While tumors become addicted to oncogenes like Ras, the microenvironment in which tumor cells reside changes during tumorigenesis; the cells are surrounded initially by normal tissue and later by tumor tissue. Hence, we asked if Ras exerts its oncogenic effects through the same set of effectors during different stages of tumorigenesis. We now show in human cells that the Ras effector pathways MAPK, RalGEF, and PI3K are required to initiate tumor growth. Conversely, activation of the PI3K/AKT pathway replaced Ras once tumors formed, although other effectors were still activated independently of Ras, presumably by factors provided upon the establishment of a tumor microenvironment. Thus, as tumorigenesis progresses the addiction of cancers to their initiating oncogene is reduced to, at least in the case of Ras, the PI3K/AKT pathway.